Conventionally, the very small metal balls used as bump cores in semiconductor packages of BGA (Ball Grid Array) type have outer diameter of about 0.1 mm.about.1.0 mm and consist of solder of predetermined composition as the material; additionally, there has recently been proposed in consideration of electrical characteristics and mechanical characteristics a chip carrier of a core material of a metal ball of Kovar (Ni--Co--Fe alloy), Cu, 42Ni--Fe alloy, or the like coated with a soldering material (Japanese Publication of Unexamined Patent Application S62-112355).
As methods for producing the aforementioned very small metal balls there have been proposed a so-called drip-into-liquid method wherein molten metal is dripped into a liquid of predetermined temperature, rendered spherical through the surface tension of the molten metal per se, and solidified as-is (Japanese Publication of Unexamined Patent Application H7-252510); so-called mechanical plastic working methods such as forming with a mold or the like (Japanese Publication of Unexamined Patent Application H4-354808); a vibrating heating method in which a metal particle or metal piece is arranged on a flat board in a non-oxidizing atmosphere, heated and melted while applying vibration thereto, rendered spherical through the surface tension thereof, and solidified as-is (Japanese Patent Publication H2-50961), and the like.
In the aforementioned drip-into-liquid method and mechanical plastic working methods, it is possible to produce very small balls of relatively large diameter, but all have poor workability and are unsuited to mass production. With the drip-into-liquid method in particular there is large deviation in dimensional precision, and to ensure very small balls having the dimensional precision required as bumps for BGA type semiconductor packages, it is necessary to separate metal balls of the desired diameter by classification, so yield is extremely poor and this is a factor contributing to increased cost.
Further, in mechanical plastic working methods, various die are required depending on ball diameter, and in addition to the aforementioned workability this is a factor contributing to increased cost.
In methods of obtaining metal balls by heating and melting in a jig as well, deviation of the diameter of the metal balls occurs as a result of deviation in individual metal wire piece unit weight; for example, in the case of a soft metal such as solder, it is difficult to obtain metal balls having identical unit weight, and therefore deviation in diameter is generated so that, like the drip-into-liquid method, there is the problem of extremely poor yield.